wallwarts and regulation..

Jay Martin jmar at intface.com
Fri Mar 6 16:49:43 CET 1998

OK OK OK!  Since I'm getting somewhat RIPPED about my response to the
power supply questions, let me clarify a few issues about using the
LM7812 instead of an adjustable regulator and why the use of a 1N914
instead of something "heftier" like a 1N400x.

First the diode issue..the 1N914 is generally OK to use for low current
devices, and that is what the gentleman mentioned in his query.  But I
will concede that the 1N400x is probably a better choice.  It was off
the top of my head..what can I say.

The other issue about using a small trimpot IN ADDITION to the diode is
another matter.  First we are only talking about a .3v drop we are
trying to add to the circuit.  Additionally we are NOT talking about a
variable load variable voltage source here.  Since the device probably
has a predictable load and the voltage we desire is a known one, a 30
turn trim pot to "shim up" the voltage would be OK.  This of course
would be subject to current demands and we would have to throw it out
the window if the load is not a fairly constant one.

Yes the LM317 is an AWESOME regulator for adjustable voltages, and it
has great stability.  But it does require a few more components to make
it that way and then all we really want is a stable voltage output...at
only one voltage mind you.  It seems that the 317 may be a bit overkill
for something that can be achieved without muss and fuss.  That's really

As for the transformer required, the voltage on the secondary could
probably be as low as 10v before rectification if full wave was used.
Now I KNOW that a heated discussion is about to begin, so let me
explain.  When dealing with AC the voltage is generally measured as the
RMS value.  Therefore a 10v secondary would actually be putting out a
14.14v peak voltage.  When full wave rectification and a good filter is
used, the voltage can easily achieve 16v DC or more (yes, it is somewhat
dependent on load factors and the current capacity of the secondary).
That give more than enough voltage to get 13v out of without adding
extra voltage that needs dissipating as HEAT.

And before you all RIP me again..real world example.  I used to sell a
home-made power supply for the commodore 64 (many many moons ago) that
used a step-down transformer rated at 9V 2A center tapped.  I would then
"crack" the windings to get 2 4.5V windings (no..you CAN'T always get
away with that either..trust me).  I would then use put one into the 12V
AC input that was internally regulated in the C64 to 12v with a 7812
(and split to a 7805 as well..used for the VIC chip) and would then send
the other winding to a full-wave rectifier and used it for the +5v DC
also required by the C64.  The power supply was dubbed the name "Marvin"
and I still have the prototype running today (here..I'll blow the dust
off and plug it in for you).  Impossible??  Not really.  The secondaries
rarely were 4.5V but measured closer to 6V (AC) even under moderate
load.  After filtering and rectification it achieved 10V.  It was then
pumped (read doubled..sort of) to around 15v.  Enough to run the 12v

Well, I'm done preaching.  Time for the alter call.  Don't everybody
throw tomatoes at once.

Jay Martin
Jmar at intface.com

> -----Original Message-----
> From:	KA4HJH [SMTP:terrymbowman at rica.net]
> Sent:	Thursday, March 05, 1998 11:28 PM
> To:	synth-diy at mailhost.bpa.nl
> Subject:	RE: wallwarts and regulation..
> Scott wrote:
> >All in all, my impression is that there are better ways.  I know
> there
> >are regulator ICs that are 'programmable' voltage wise and can be
> >beefed up with a pass transistor for higher currents.  This will
> >result in a higher component count, but also in a better regulator.
> >I'd hate to introduce problems into a system using a questionable PS
> >design that could be eliminated with an extra 2 hours of work and a
> >few extra dollars worth of parts.
> Amen to that sentiment. Now, I am certainly NOT an EE, as was said
> recently
> in this forum (by somebody else but without the emphsis), but while
> this is
> a classic example of a situation where there are all kinds of
> interesting
> solutions worth chatting about, one of which may actually save or have
> saved your butt in a pinch one day, THIS project cries out for a
> simple
> LM317 and two resistors.
> And for heaven's sake don't hook a pot up to an LM7805 and think you
> have
> an adjustable regulator. It will regulate under load but not with
> respect
> to the unregulated DC. I've tried it on the bench; it doesn't work.
> According to the databooks I've seen, there has to be a current buffer
> between the output feedback and the voltage divider, ie, an op-amp.
> The
> LM317/319 don't need any extra components, so use them or the proper
> fixed
> voltage regulator instead. KISS.
> I mention the latter case because a certain well-known PS kit (which I
> purchased years ago and will remain nameles--although I think it's
> still in
> production) is designed just this way--7805/7905's turned into a
> variable
> regulators with pots. I took it too work and everyone groaned. Like I
> said,
> we tested it under real-world conditions and it didn't regulate with a
> varying DC supply. The solution: I substituted 7815/7915's instead and
> it
> worked fine. Rock steady.
> "Sometimes the simplest solution is the best solution--unless you
> con't
> figure out how to get rid of the body."
> Terry Bowman, KA4HJH

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